Really? It is my understanding that if the URL matches the certificate Subject, which is one of the purposes of a Certificate Subject, these errors won't pop up. Note: True this is not a date issue, but there are other aspects of certs that may suggest inappropriate sources like the error here.
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user138997Mar 9 '13 at 21:45

It appears that the hostname and the subject don't match. This is a source of suspicion of invalid certs. In essence I'd hope that Canonical would produce certificates with a subject specifying Ubuntu not Canonical. That would clear up these errors.
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user138997Mar 9 '13 at 21:51